Methodology
Our calculation engine.
Where the numbers come from
Each item in the catalogue stores a saving in watts, the speed that saving was measured at, and a source. Most come from published wind tunnel and rolling resistance testing. Some are estimates, and those are labelled as estimates with a low confidence score. Every figure in the tool shows its source and confidence in the ⓘ tooltip. The full list is on the Sources page.
Savings are scaled to your speed
A saving measured at 45 km/h is not the same saving at 32 km/h. Aerodynamic drag rises with the cube of speed, so aero savings are converted to a drag area figure and recalculated at whatever speed you set. Rolling resistance and drivetrain losses scale with speed directly, not cubed, so they’re handled separately. Change the speed slider and the ranking changes with it.
Everything is measured from your setup, not from scratch
The tool asks what you’re running so it can price the upgrade rather than the item. If you already have an aero road helmet, a TT helmet is worth about 4 watts to you, not the 8 it would be worth to someone in a vented road helmet. This is why step 2 matters, and why it’s worth correcting anything the photo got wrong.
Savings don’t simply add up
Two aero changes rarely give you the sum of both. Cleaning up the airflow in one place reduces what there is to gain elsewhere. Any combined total applies a diminishing returns discount rather than adding the figures together.
Some things also can’t be combined at all. You can’t fit latex tubes and go tubeless. You’d buy either a disc wheel or a disc cover, not both. Where items are alternatives, totals count only the best one. The list still shows you the options, because choosing between a £800 wheel and a £1,200 one is your call, not ours.
What a wind tunnel can actually resolve
A controlled wind tunnel session resolves to roughly ±2.5 watts at 40 km/h. Anything in the list smaller than that sits below what testing can reliably separate. Those rows are worth treating as directional rather than precise, and that’s why they rank low.
Position advice has no watt figures
We are still testing our position model. Getting this right needs a real relationship between body angles and drag area, which we don’t have yet. Until then, position advice describes what to try and says nothing about watts.
What the tool doesn’t account for
- Yaw. Wheel performance varies a lot with crosswind angle, and we use a single figure.
- Rider size. A bigger rider has more to gain from most aero changes.
- Tyre width and pressure beyond a single figure per tyre tier.
- Whether you can actually hold a position for the length of your event.